Fix XML syntax file to highlight tag names according to the standard

I just noticed that the dex syntax highlighter shows dash characters in XML tag names as errors; even though they're valid according to the spec and are quite common in Android XML files.

Relevant sections of the spec:

* http://www.w3.org/TR/REC-xml/#d0e804
* http://www.w3.org/TR/REC-xml/#sec-starttags

This commit bring a few other small details in line with the spec, although it omits Unicode ranges.